1. Field of the Invention
The present invention generally relates to magnetic recording media, methods of producing the same and magnetic storage apparatuses, and more particularly to a magnetic recording medium that includes a recording layer having Co as a main component and crystally oriented in a predetermined direction, a method of producing such a magnetic recording medium, and a magnetic storage apparatus using such a magnetic recording medium.
2. Description of the Related Art
Recently, magnetic storage apparatuses that are provided in personal computers and dynamic image recording apparatuses for home use, such as magnetic disk drives, have large storage capacities that exceed 100 GB, mainly for the purpose of recording dynamic images. The demands for even larger storage capacities and lower costs of the magnetic disk drives are likely to increase.
Presently, utilizing the in-plane (or longitudinal) magnetic recording technique in the magnetic disk drive, the recording density has been increased considerably to increase the storage capacity. Further, the signal-to-noise (S/N) ratio of the magnetic disk has been improved, and the sensitivity of the magnetic head has been increased. As a result, it has become possible to realize an in-plane recording density that exceeds 100 Gbit/in2.
In the conventional in-plane magnetic recording technique and perpendicular magnetic recording technique, it is necessary to reduce the size of the crystal grains of the ferromagnetic material forming the recording layer, in order to further improve the S/N ratio at high recording densities. However, when the crystal grain size is reduced, the magnitude of the magnetization recorded in the recording layer, that is, the remanent magnetization, gradually decreases with lapse of time. In other words, the thermal stability of the recorded magnetization decreases. In order to solve this problem, a ferromagnetic material having a large anisotropic field for the recording layer can be used.
However, a new problem is introduced in that the recording becomes more difficult, that is, the recording performance deteriorates. The larger the anisotropic field of the ferromagnetic material, the larger the recording magnetic write field intensity needs to be. Moreover, the recording magnetic field intensity that can be generated by the magnetic head is limited by the material used for the write head magnetic pole. For this reason, the ferromagnetic material that can be used for the recording layer also becomes limited, and sets the limit to the achievable recording density.
In another approach to increase the recording density, a magnetic recording medium having a recording layer with the axis of easy magnetization inclined by 45 degrees with respect to the substrate surface has been theoretically proposed in Kai-Zhong Gao and H. Neal Bertram, “Magnetic Recording Configuration for Densities Beyond 1 Tb/in2 and Data Rates Beyond 1 Gb/s”, IEEE Transactions on Magnetics, Vol. 38, No. 6, November 2002. According to this first proposed magnetic recording medium, the angle formed by the axis of easy magnetization of the recording layer and the direction of the recording magnetic field is selected, so as to reduce the recording magnetic field intensity that is required to reverse the magnetization. In addition, by using this first proposed magnetic recording medium, it is expected that a high recording density will be realized for both the in-plane magnetic recording technique and the perpendicular magnetic recording technique. In this first proposed magnetic recording medium, it is required that the inclination direction of the magnetization of the columnar crystal grains of the recording layer is uniformly oriented in a track longitudinal direction or a track width direction. The S/N ratio of this first proposed magnetic recording medium is expected to be higher than 12 dB from the conventional perpendicular magnetic recording medium, at the recording density of 1 Tbit/in2. This improvement in the S/N ratio is made possible by the fact that the anisotropic field of the recording layer can further be improved than that of the conventional perpendicular magnetic recording medium.
For example, a Japanese Laid-Open Patent Application No. 2004-227745 proposes a magnetic recording medium having at least a soft magnetic back layer and a recording layer, wherein the axis of easy magnetization of the recording layer inclined with respect to a normal to the magnetic recording medium. High resolution and S/N ratio are expected of this second proposed magnetic recording medium, without deteriorating the thermal stability of the recorded magnetization. When forming the recording layer of this second proposed magnetic recording medium, a mask plate for limiting incident grains is provided between a target and a substrate, so that only the incident grains from a direction inclined in one direction with respect to the normal to the substrate are deposited.
However, the first proposed magnetic recording medium is proposed in theory only, and Kai-Zhong Gao and H. Neal Bertram do not disclose a particular method of realizing the magnetic recording medium in which the axis of easy magnetization of the recording layer is inclined by 45 degrees with respect to the substrate surface.
On the other hand, the second proposed magnetic recording medium requires a deposition apparatus to be modified, which consequently increases the production cost. In addition, the crystal growing direction is inclined with respect to the direction of the normal to the substrate of the second proposed magnetic recording medium, by controlling the incident direction of the incident grains. The direction of the axis of easy magnetization becomes the crystal growing direction. In other words, it is difficult to control the direction of the axis of easy magnetization, because the direction of the axis of easy magnetization is controlled by the incident direction of the incident grains.